A problem in elemental mass spectrometry is the presence of polyatomic and multicharged ion interferences having the same masses as isotopes to be measured. For example, in plasmas sustained in argon, argon-based interfering ions such as Ar+, Ar2+, ArO+, ArOH+ have masses that overlap with the masses of isotopes of Ca, Fe, Se, which makes it difficult to produce reliable analytical results for trace amounts of such isotopes.
Known methods for attenuating interfering polyatomic or multicharged ions have involved promoting reactive (that is, ion-molecule charge transfer reactions) and collisional decomposition of the interferences via the use of mixed gas plasmas, such as the addition of hydrogen to the argon conventionally used in ICP-MS, and the use of various collision or reaction cells that may contain selected reactive or collision gases. It is also known to promote reactive (charge transfer) and collisional decomposition of interfering ions in the region of the interface between a plasma ion source and mass analyser, for example in the region of the sampling-skimmer cone interface in an inductively coupled plasma mass spectrometer (ICP-MS). For example, as long ago as 1986 R. S. Houk and colleagues listed “adding a foreign gas (e.g. Xe) into the ICP or vacuum system to react with and remove the undesired ion” as one approach to solving the problem of spectral overlap interferences in ICP-MS. (R. S. Houk, J. S. Crain, and J. T. Rowan, “What can be done about spectral overlap interferences in ICP-MS”, Abstracts, 1986 Winter Conference on Plasma Spectrochemistry, Kailua-Kona, Hi., USA, Jan. 2-8, 1986, p. 35). For another example U.S. Pat. No. 4,948,962 entitled “Plasma Ion Source Mass Spectrometer” in the name of Yasuhiro Mitsui et al discloses introducing a suitable gas into the first differential pumping region between the sampling and skimmer cones of an ICP-MS to promote charge transfer reactions. However it teaches repulsion of electrons from the plasma prior to the charge transfer reaction region, for example by use of a negatively charged mesh grid immediately behind the sampling cone orifice. Thus it teaches the introduction of a reactive gas into a region through which what is effectively an ion beam extracted from the plasma passes. U.S. Pat. No. 6,259,091 entitled “Apparatus for Reduction of Selected Ion Intensities in Confined Ion Beams” by Gregory C Eiden et al discloses introducing a reactive gas almost immediately behind the skimmer cone orifice. As in U.S. Pat. No. 4,948,962, this is in the region of an extracted ion beam and this beam must collide with the introduced gas molecules to undergo the necessary reactions. A dilemma with this is that the analyte signal intensity is reduced by collisions, thus maximum analyte sensitivity requires minimum collisions, but efficient attenuation of interferences requires maximum collisions. This dilemma unavoidably comprises the efficiency of these prior methods. U.S. Pat. No. 6,259,091 also discloses use of a reaction cell containing the reactive gas, namely hydrogen. The reactive gas in this cell is maintained at an optimal pressure, but the cell is located wholly within a vacuum region at a different pressure, which complicates operation of this arrangement.
The discussion herein of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date established by the present application.
An object of the present invention is to provide a plasma mass spectrometry instrument and method for elemental and isotopic analysis in which the attenuation of interfering polyatomic and multicharged ions is improved.